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what causes high idle?

What are all the possibilities that could cause high idle. It doesn’t happen all the time, but sometimes the XJ has a high idle. While idling high, I typically pop the hood and manually adjust the throttle to bring the idle back down. However, sometimes it tries to kick back up. Thoughts?
 
Dude--year, engine?
 
The Idle Air Control (IAC) is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality
 
Not as common as an air leak, but a leaking fuel injector would do it too.

Most common is an air leak, in any vaccum line, intake manifold leak, or sticking IAC. Exhaust leaks near the O2 sensor have also been reported to cause high idles. The TPS can also cause it if it is on its last legs, and it is giving inconsistent idle position resistance. Lastly bad wiring, battery, positive or grounds can add to the engine speed issues.
 
Thanks everyone for all your responses. It’s obvious that you guys have done a lot of research and have the passion to understand how your XJ works. My last UOA showed fuel and traces of coolant in the oil, so I’m pretty much milking this thing along. Overall, the motor seems to run good with no overheating problems. I’m at 175,000 miles now, which isn’t really high mileage for the 4.0L, so I’m torn between putting money into this motor, or just saving up for a new motor.
 
I would guess that the IAC is sticking. On my 2000, I can create a huge vacuum leak by disconnecting the power brake vacuum line, and the system is smart enough to bring the idle right back down in a few seconds.
 
I would guess that the IAC is sticking. On my 2000, I can create a huge vacuum leak by disconnecting the power brake vacuum line, and the system is smart enough to bring the idle right back down in a few seconds.

That’s good to know, I’ll probably start with the IAC first.
 
Really, other than the 0331 head issue the 4.0 is a tough cookie and can go 300+ with regular maintenance.

Depending on your needs I would suggest you save up for a new head and drive it until the problem really brews up.
 
Since the oil analysis showed fuel in the oil, that says it is highly probably you have a leaking or sticking fuel injector. If it gets bad enough it will ruin the compression quickly on that cylinder!!!!! Or you have a bad plug(s) or wire(s) that are not getting a hot enough spark on one or more cylinders.

I would pull the injectors and clean and test them. Same for the spark plugs, wires, and the cap and rotor!!!!!

Small coolant in the oil leaks may be reduced by adding bars leaks brown fiber pellets to the coolant. They are dry, you crumble them and add them to the radiator.

If too much unburned fuel is getting past the combustion chamber a good, working O2 sensor might also see too little O2, and compensate by adding O2, thus resulting in an increased idle.
 
Depending on your needs I would suggest you save up for a new head and drive it until the problem really brews up.

I’m saving $300 a month for a new engine, new injectors, new water pump, new alternator, new battery, new drive belt, new hoses, new aluminum 3 core radiator, new upper and lower hoses, new thermostat, new heater core, new engine mounts, new starter, new coil packs, new plugs, new injectors, new header, new front pipe, new converter, new catback, and new sensors. I‘m budgeting $5,000 for parts and labor. So I’ve got a long time (17 months) to go before I can fully fund and execute this project. This gives me plenty of time to buy parts when I can find a good buy.
 
Since the oil analysis showed fuel in the oil, that says it is highly probably you have a leaking or sticking fuel injector. If it gets bad enough it will ruin the compression quickly on that cylinder!!!!! Or you have a bad plug(s) or wire(s) that are not getting a hot enough spark on one or more cylinders.

I would pull the injectors and clean and test them. Same for the spark plugs, wires, and the cap and rotor!!!!!

Small coolant in the oil leaks may be reduced by adding bars leaks brown fiber pellets to the coolant. They are dry, you crumble them and add them to the radiator.

If too much unburned fuel is getting past the combustion chamber a good, working O2 sensor might also see too little O2, and compensate by adding O2, thus resulting in an increased idle.

Here's my first report:

Unit ID: 2000 Jeep Cherokee
Second ID: Inline 6
Unit Type: Unleaded Gasoline Engine
Application: Transportation
Unit Mileage: 171,673
Lubricant Mileage: 2,113
Lubricant: Amsoil XLT SAE Synthetic 10W30


OVERALL SEVERITY OF REPORT: (High end of ABNORMAL)

SUGGEST performing cooling system pressure check. Lubricant and filter change is suggested if not done at sampling time. Coolant indicators (Sodium, Potassium) are at a significant level. Flagged additive levels are different than what should be present for the lubricant that is identified for this unit. Please provide unit model number so we may compare data to the proper standards for this unit; Replace oil filter and top off system with fresh make-up oil if not done when oil sample was taken. Re-sample in 3,500 miles or 65 hours. Resample in 30 days.

Areas that are flagged:

CONTAMINANT METALS:
Sodium: 41 PPM (High end of NORMAL)
Potassium: 197 PPM (High end of ABNORMAL)

MULTI-SOURCE METALS:
Molybdenum: 8 PPM (Low end of ABNORMAL)
Manganese: 27 PPM (High end of NORMAL)
Boron: 41 PPM (Low end of ABNORMAL)

ADDITIVE METALS:
Magnesium: 584 PPM (Low end of ABNORMAL)
Zinc: 1063 PPM (Low end of ABNORMAL)

Here is my second report:

Unit Mileage: 174,257
Lubricant Mileage: 2,358
Lubricant: Amsoil XLT SAE Synthetic 10W30

OVERALL SEVERITY OF REPORT: (Critical)

Check for source of FUEL LEAK (injectors, lines, etc.). Fuel is at a SIGNIFICANT LEVEL; Lubricant and filter change is suggested if not done at sampling time; FUEL DILUTION has caused viscosity to decrease SEVERELY; MODERATE POTASSIUM level could be a coolant indicator; Coolant leaks at this level will most likely not be detectable through normal diagnostics; Suggest MONITORING COOLANT LEVEL closely between samples; Magnesium is slightly high for this lubricant; Please provide until model number so we can compare data to the proper standards for this unit; Resample at half intervals.

CONTAMINANT METALS:
Sodium: 14 PPM (NORMAL)
Potassium: 45 PPM (Low end of ABNORMAL)

MULTI-SOURCE METALS:
Molybdenum: 36 PPM (NORMAL)
Manganese: 6 PPM (NORMAL)
Boron: 198 PPM (NORMAL)

ADDITIVE METALS:
Magnesium: 92 PPM (Low end of ABNORMAL)
Zinc: 757 PPM (NORMAL)

Fuel: 3.6% (High end of ABNORMAL)
Viscosity 9.0 (Critical)
 
I would pull the injectors and clean and test them, or have some one else clean and test them. I have found that a 9 volt battery and a simple air test (blow through the injector like you are blowing up a balloon works for large leaks and simple injector tests). Or just put in new injectors (buy them now, and use them for testing) now while you still have an engine!!!!!

Change the oil, then run the engine a few minutes. Shut down and then run compression tests on each cylinder to see how much damage there is compression wise, if any.

Is it using oil yet? If so how fast? Did oil consumption change between the two tests?

I would not be too worried about a coolant issue reading all that data. I would just flush the coolant system, and change the coolant, and add maybe one brown pellet (Bar's Leaks http://www.barsproducts.com/HDC.htm ) the one I described below to the coolant. If you caught it soon enough and change the injectors immediately you may be able to save the engine.

I am not sure Amsoil isn't using some potassium in the oil? Not sure it is a sign of coolant all the time? I think I saw where some oils were using potassium, and some coolants use only sodium, not potassium. Are you using any oil additives, or gas additives at all?

All that gas in the oil may have broken loose and dissolved some trash, old engine deposits, and artificially changed some of those metals numbers by dissolving and grinding up old loosened deposits. May not be all just metal wear if you caught it early enough. I would be worried about the oil pump and oil pump screen if a lot of trash came loose!

Fact that sodium and potassium dropped is encouraging on the coolant leak issue side of the equation!

Just curious, why don't they test for other metals? Like iron, chrome, nickel, etc?
 
As Ecomike has brought out, the main problem seems to be fuel contamination of your oil. Have you studied your spark plugs? They should be mostly whitish with possible light tan color. If one or a couple of plugs are darker than the others, suspect that cylinder (injector, spark plug, compression, etc.).

While you have the plugs out, run a compression test. That is the best indicator of engine condition (notwithstanding bearing wear).

One other point that I don't believe has been touched on is operating temperature. Are you running at least 195 temperature? Not doing so can result in your oil contamination also.
 
All good points that I missed. I mentioned it earlier down below, but yes, a weak or no spark to one or more cylinders would cause the fuel in the oil too, not just an injector leaking. X2 on pulling the plugs and inspecting each for the problem child cylinder!!! Then consider the spark plug, HV SP wire, low compression, bad valves in that cylinder and the injector as all possible sources of the problem. Compression test is great idea. SP wire is easy to test with an ohm meter.

Also pulling each SP wire one at a time, and the injector wires one at a time while running the engine can also isolate a cylinder that is not firing properly.

Lastly, since this is an OBD-II jeep, a good OBD-II scanner should be very revealing as to fuel trim issues on any one cylinder, among other things like engine coolant temp!!!!!!! Many parts store do free scans. Not sure which have the best scanners, all scanners are not equal!!!! Some tell you a lot more than others do!

As Ecomike has brought out, the main problem seems to be fuel contamination of your oil. Have you studied your spark plugs? They should be mostly whitish with possible light tan color. If one or a couple of plugs are darker than the others, suspect that cylinder (injector, spark plug, compression, etc.).

While you have the plugs out, run a compression test. That is the best indicator of engine condition (notwithstanding bearing wear).

One other point that I don't believe has been touched on is operating temperature. Are you running at least 195 temperature? Not doing so can result in your oil contamination also.
 
Or just put in new injectors (buy them now, and use them for testing) now while you still have an engine!!!!!
Fair enough, where is the best place to buy new injectors. I will need them for the new engine anyway, so I don't mind buying new injectors.

Is it using oil yet? If so how fast? Did oil consumption change between the two tests?

So far, it's not using oil.

I would not be too worried about a coolant issue reading all that data. I would just flush the coolant system, and change the coolant, and add maybe one brown pellet (Bar's Leaks http://www.barsproducts.com/HDC.htm ) the one I described below to the coolant.

Yeah, I'm not worried about the coolant either, it's minor and not showing "critical" in my UOA's like the fuel is. I've already done a coolant flush. I may add the Bar's brown pellet as you recommended.

If you caught it soon enough and change the injectors immediately you may be able to save the engine.

The XJ was very neglected when I bought it. To be honest, I mainly bought it because it had a rust free body and was drivable. When I first bought it, the overdrive didn't work in it. I did an Amsoil ATF drain and fill about 3 times and it started working. Now she shifts fine. I plan on buying a new motor regardless, but it would be nice if I could save this engine as it would allow me to put money elsewhere before I have to put money into a new motor.

I am not sure Amsoil isn't using some potassium in the oil? Not sure it is a sign of coolant all the time? I think I saw where some oils were using potassium, and some coolants use only sodium, not potassium. Are you using any oil additives, or gas additives at all?

I'm not sure about potassium in the oil, but I do know that OIL ANALYZERS are very familiar with the different additives that Amsoil uses. I'm not using any oil additives; however, I'm using HEET in my gasoline.

All that gas in the oil may have broken loose and dissolved some trash, old engine deposits, and artificially changed some of those metals numbers by dissolving and grinding up old loosened deposits. May not be all just metal wear if you caught it early enough. I would be worried about the oil pump and oil pump screen if a lot of trash came loose!

Yeah, I thought about this also, which scares me more than anything. The oil came out much darker this time than ever before, which may indicate that the gasoline was doing some cleaning. I wish I had my 2 micron by-pass filter installed, but I'm going to have to move the battery to the back of the Jeep to have room to install my dual remote bypass filter. The Amsoil full flow filter is a good filter, so I'm sure it's doing its job to capture as much debris as possible.

Fact that sodium and potassium dropped is encouraging on the coolant leak issue side of the equation!
Yes, that is one less headache. LOL

Just curious, why don't they test for other metals? Like iron, chrome, nickel, etc?

They do test for all the other metals, but I only listed the things that were out of "normal" range. Everything else was "normal".
 
As Ecomike has brought out, the main problem seems to be fuel contamination of your oil. Have you studied your spark plugs? They should be mostly whitish with possible light tan color. If one or a couple of plugs are darker than the others, suspect that cylinder (injector, spark plug, compression, etc.). While you have the plugs out, run a compression test. That is the best indicator of engine condition (notwithstanding bearing wear).

I put new plugs in about a year ago. I will have the local shop pull the plugs and check compression.

One other point that I don't believe has been touched on is operating temperature. Are you running at least 195 temperature? Not doing so can result in your oil contamination also.

I'm running a new thermostat which was changed when I did my coolant flush about 8 months ago. I'm running a Stant Superstat 195.
 
Also pulling each SP wire one at a time, and the injector wires one at a time while running the engine can also isolate a cylinder that is not firing properly.

Remember, I don't have plug wires, LOL. I will have each coil pack checked.

Lastly, since this is an OBD-II jeep, a good OBD-II scanner should be very revealing as to fuel trim issues on any one cylinder, among other things like engine coolant temp!!!!!!! Many parts store do free scans. Not sure which have the best scanners, all scanners are not equal!!!! Some tell you a lot more than others do!

That's not a bad idea. I just need to find a good shop. I don't mind shelling out the money if I know it's a good shop. I live 40 miles from the nearest city, so I don't have easy access to a lot of shops. We do have a local shop in a village nearby, but they are expensive (for what you get) and a bit of a shade-tree type of operation. Maybe I should take it into a Jeep dealership and see if they can diagnose and repair quickly.
 
If it is not using any oil, and running on synthetic 3% gasoline you have no worries! LOL. I tired synthetic on my 278,000 mile 87, and could not pour it in fast enough!!!!LOL

http://www.fiveomotorsport.com/product-search/?v=19036

Is where many of us buy new injectors.

The problem with chunks of junk coming loose is the oil going through the oil pump is not filtered on the way in, only screened! And if a bunch of hard carbon came loose it can clog the oil pump inlet screen, and what gets through the screen can trash the oil pump, and do engine damage if it seizes. I lost my only broken oil pump, and a little later engine, that way, 78 Dodge V-8, from gas getting in the oil of a 350,000 mile engine.

I see you just mentioned the new spark plugs. Keep in mind even a brand new plug, or other parts can be bad!!!

I bought a half decent OBD-II scanner at Walmart 2 years ago, for about $120. They have gotten cheaper, and I suspect if you shop online (check Ebay also) that you could get one that shows live data for stuff like fuel trim, O2 sensor data, coolant temp....... for under $200 easy! Might be worth it for you in the long run!

I would be VERY afraid to drive it 40 miles with 3% gasoline in the oil!!!!!
 
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